... 4 Da, the decrease of the molecular mass value. The most proba-ble reason for a 2 Da decrease is the formation of anS–S bond; although this was totally unexpected and unprecedented, the top-down ... Neil Kelleher,Harold Scheraga and Klaas van Wyck for valuablediscussions, and the General Medical Institute of the National Institutes of Health, GM16609, for generousfinancial support.References1 ... causes a 1 Da increase in any fragment ion containing that residue. The average mass gain of N-terminal and C-terminal fragment ions are plottedas positive and negative, respectively, mass increases,...